This invention relates to a solid fuel burner for a rotary kiln and particularly relates to a direct coal firing system using a burner with a multi-barrel construction.
In a direct coal firing system, hot air is drawn from a rotary kiln hood and is used to dry coal before it is sent to a mill for pulverization. The pulverized coal is entrained in a primary airflow for conveying the coal to a burner, the airflow also supplying 35 to 45% of stoichiometric air requirements for combustion of the coal. The balance of the stoichiometric requirements is supplied by a super heated secondary air flow contained by a kiln hood at the discharge end of the rotary kiln.
In indirect firing systems, pulverized coal is separated from the air required in the pulverizing mill using a cyclone or the like and the coal is stored so that it can be withdrawn as and when required. Coal and primary air firing rates are independent of the coal pulverizing mill and the volume of relatively cool primary air used to convey the coal to the burner is kept to a minimum so as not to exceed 20% of stoichiometric air requirements and thereby improve burner efficiency. Because of improved efficiency, indirect firing systems are generally preferred over direct firing systems and new rotary kiln installations usually include the infrastructure required to support an indirect coal firing system.
While many improvements have been made to burners for indirectly fired rotary kiln installation to still further increase efficiencies, very little effort has been devoted to improving the efficiency of direct fired rotary kiln installations, many of which continue to operate throughout the world at margins which cannot justify the cost of modifying the installations to accommodate the additional equipment required to support an indirect fired burner system.
Some improvements to indirect fired burner systems include pre-heating primary air with recirculated flue gas as in U.S. Pat. No. 4,387,654. In U.S. Pat. No. 4,206,712, a multi-barrel burner construction with a tertiary warm air supply for preheating a coarse coal fraction is disclosed. A small quantity of high-pressure, high velocity control air is directed at the fuel stream through a swirling air annulus to create and maintain a turbulent region around the tip of the burner nozzle. The quantity of air is varied from between 5 to 15% of stoichiometric air requirements.
In U.S. Pat. No. 4,838,185, a cool air jacket defined by a third barrel in the burner is provided to isolate a swirling jet of burning gases from the wall of the combustion chamber so that combustion can take place without damage to its walls and in U.S. Pat. No. 5,299,512, it is submitted that it is preferable to introduce primary air, in an axial direction to avoid rotation of the primary air stream altogether.
There are thus widely divergent views on whether spiral air deliveries are desirable in indirect coal fired burners and since the volumes of primary air fed into an indirect fired burner are significantly lower than those required in direct fired burners, the fluid flows and dynamics also differ considerably. Any teachings found in these prior art devices are clearly inappropriate for use in direct coal fired burners.
An object of this invention is to provide a system which can easily be integrated into existing direct coal fired burner installations to improve their efficiency without any significant capital investments.
In accordance with the invention, there is provided a direct coal fired burner system for a rotary kiln in which the burner has a multi-barrel construction through which primary air for conveying coal is heated with tertiary air sourced from the rotary kiln, and the shape of the emerging flame is controlled by a first annulus of swirling tertiary air adjacent the primary airstream and by a second annulus of laminar tertiary air defining an outer envelope for the flame. The volume and relative velocities of air in the tertiary air supplies are all controllable through the use of respective dampers provided in tertiary air supply conduits.